The quantitation of carbamino adduct formation of angiotensin II and bradykinin

Equilibrium Dynamics of β-N-Methylamino-L-Alanine (BMAA) and Its Carbamate Adducts at Physiological Conditions

Elevated incidences of Amyotrophic Lateral Sclerosis/Parkinsonism Dementia complex (ALS/PDC) is associated with β-methylamino-L-alanine (BMAA), a non-protein amino acid. In particular, the native Chamorro people living in the island of Guam were exposed to BMAA by consuming a diet based on the cycad seeds. Carbamylated forms of BMAA are glutamate analogues. The mechanism of neurotoxicity of the BMAA is not completely understood, and BMAA acting as a glutamate receptor agonist may lead to excitotoxicity that interferes with glutamate transport systems. Though the interaction of BMAA with bicarbonate is known to produce carbamate adducts, here we demonstrate that BMAA and its primary and secondary adducts coexist in solution and undergoes a chemical exchange among them. Furthermore, we determined the rates of formation/cleavage of the carbamate adducts under equilibrium conditions using two-dimensional proton exchange NMR spectroscopy (EXSY). The coexistence of the multiple forms of BMAA at physiological conditions adds to the complexity of the mechanisms by which BMAA functions as a neurotoxin.

Characterization of the redox transition of the XRCC1 N-terminal domain

XRCC1, a scaffold protein involved in DNA repair, contains an N-terminal domain (X1NTD) that interacts specifically with DNA polymerase β. It was recently discovered that X1NTD contains a disulfide switch that allows it to adopt either of two metamorphic structures. In the present study, we demonstrate that formation of an N-terminal proline carbimate adduct resulting from the nonenzymatic reaction of Pro2 with CO2 is essential for stabilizing the oxidized structure, X1NTDox. The kinetic response of X1NTDred to H2O2, monitored by NMR, was determined to be very slow, consistent with involvement of the buried, kinetically trapped Cys12 residue, but was significantly accelerated by addition of protein disulfide isomerase or by Cu(2+). NMR analysis of a sample containing the pol β polymerase domain, and both the reduced and oxidized forms of X1NTD, indicates that the oxidized form binds to the enzyme 25-fold more tightly than the reduced form.

Carbamino group formation with peptides and proteins studied by mass spectrometry

At high pH and in the presence of dissolved CO(2), the N-terminus and epsilon-amino groups of amino acids, peptides, and proteins can form carbamino adducts with CO(2), R-NH(2) + CO(2) <--> R-NHCOO(-) + H(+). We report the first study of carbamino group formation by electrospray ionization (ESI) mass spectrometry (MS). Angiotensin II, bradykinin, substance P, and insulin have been studied. A careful optimization of the instrumental parameters was necessary to allow the transfer of the fragile adducts into vacuum for mass analysis. Particularly, dissociation of the adducts in the ion sampling process and pH changes in ESI must be minimized. With these precautions, levels of carbamino group formation of angiotensin II and bradykinin determined from mass spectra agree with those expected to be in solution, calculated from literature equilibrium constants. Thus, ESI MS can quantitatively measure ratios of carbamino adduct to total peptide concentration in solution. Values of equilibrium constants for carbamino group formation with substance P (pK(c) = 4.77 +/- 0.18) and insulin (pK(c) = 4.99 +/- 0.05) are reported for the first time.

Characterization of the carbamino adducts of insulin

Carbon-13 (13C) nuclear magnetic resonance spectroscopy (NMR) is performed to characterize the formation of carbamino adducts between insulin and (13C) carbon dioxide over a range of pH values in the presence of a physiological concentration (23 mM) of sodium bicarbonate. The peaks from two of the carbamino adducts resonate at higher frequencies than the signal from bicarbonate, at 164.6 and 165.3 ppm, and are attributed to the adducts with the terminal amino groups of phenylalanine B1 and glycine A1. The intensities of these signals vary with the pH, with unique patterns. Over 6% of each terminal amino group exists as the carbamino adduct at the optimum pH values of 7.8 and 8.3. A unique third adduct resonates at 159.3 ppm, and is attributed to lysine B29. This adduct is present on 2% of the insulin molecules at pH 8.2, but has minimal intensity at pH 7.4. No signals from adducts are detected below pH 6.2, where the amino groups exist predominantly in the protonated form. Creation of the adducts is rapid and they are stable for over 4 wk at 37 degrees C. The narrow bandwidth of the resonance of the adduct (4.0-4.5 Hz) relative to the irreversible cyanate adduct is consistent with molecular forms of the carbamino adduct smaller than the 2-Zn-hexamer which is the preponderate form of clinically utilized U-100 insulin (i.e., 100 U/ml).